On-disk files in a container are ephemeral, which presents some problems for non-trivial applications when running in containers. One problem is the loss of files when a container crashes. The kubelet restarts the container but with a clean state. A second problem occurs when sharing files between containers running together in a Pod. The Kubernetes volume abstraction solves both of these problems. Familiarity with Pods is suggested.
Background Docker has a concept of volumes, though it is somewhat looser and less managed.